package com.zhiche.wms.core.utils;

/**
 * 改良twitter的snowflake算法
 * Created by qichao on 2018/2/11.
 */
public class SnowFlakeId {
	/**
	 * 起始的时间戳(2018-2-11),此参数不提供客户端修改接口
	 */
	private final static long START_STMP = 1518334444697L;

	/**
	 * 每一部分占用的位数
	 */
	private final static long SEQUENCE_BIT = 12; //序列号占用的位数
	private final static long MACHINE_BIT = 5;  //机器标识占用的位数
	private final static long DATACENTER_BIT = 5;//数据中心占用的位数

	/**
	 * 每一部分的最大值
	 */
	private final static long MAX_DATACENTER_NUM = -1L ^ (-1L << DATACENTER_BIT);
	private final static long MAX_MACHINE_NUM = -1L ^ (-1L << MACHINE_BIT);
	private final static long MAX_SEQUENCE = -1L ^ (-1L << SEQUENCE_BIT);

	/**
	 * 每一部分向左的位移
	 */
	private final static long MACHINE_LEFT = SEQUENCE_BIT;
	private final static long DATACENTER_LEFT = SEQUENCE_BIT + MACHINE_BIT;
	private final static long TIMESTMP_LEFT = DATACENTER_LEFT + DATACENTER_BIT;

	private long datacenterId;  //数据中心
	private long machineId;    //机器标识
	private long sequence = 0L; //序列号
	private long lastStmp = -1L;//上一次时间戳

	public SnowFlakeId(long datacenterId, long machineId) {
		if (datacenterId > MAX_DATACENTER_NUM || datacenterId < 0) {
			throw new IllegalArgumentException("datacenterId can't be greater than MAX_DATACENTER_NUM or less than 0");
		}
		if (machineId > MAX_MACHINE_NUM || machineId < 0) {
			throw new IllegalArgumentException("machineId can't be greater than MAX_MACHINE_NUM or less than 0");
		}
		this.datacenterId = datacenterId;
		this.machineId = machineId;
	}

	/**
	 * 产生下一个ID
	 *
	 * @return
	 */
	public synchronized long nextId() {
		long currStmp = getNewstmp();
		if (currStmp < lastStmp) {
			throw new RuntimeException("Clock moved backwards.  Refusing to generate id");
		}

		if (currStmp == lastStmp) {
			//相同毫秒内，序列号自增
			sequence = (sequence + 1) & MAX_SEQUENCE;
			//同一毫秒的序列数已经达到最大
			if (sequence == 0L) {
				currStmp = getNextMill();
			}
		} else {
			//不同毫秒内，序列号置为0
			sequence = 0L;
		}

		lastStmp = currStmp;

		return (currStmp - START_STMP) << TIMESTMP_LEFT //时间戳部分
				| datacenterId << DATACENTER_LEFT      //数据中心部分
				| machineId << MACHINE_LEFT            //机器标识部分
				| sequence;                            //序列号部分
	}

	private long getNextMill() {
		long mill = getNewstmp();
		while (mill <= lastStmp) {
			mill = getNewstmp();
		}
		return mill;
	}

	private long getNewstmp() {
		return System.currentTimeMillis();
	}
}
